1. Field of the Invention
The present invention relates to a polypropylene resin composition excellent in coating properties and more particularly to an impact-resistant polypropylene resin composition preferably used in the production of exterior equipment for automobiles such as bumpers, a method for coating a molded article formed from the polypropylene resin composition, in particular, exterior equipment for automobiles such as bumpers, and the coated molded article.
2. Description of the Prior Art
Polypropylene resins have a wide variety of applications because of low density, high rigidity, high heat resistance and high resistance to chemicals. However, it is known that the polypropylene resins are essentially non-polar and accordingly, inferior in coating properties. For instance, when polypropylene bumpers for automobiles are coated, it is necessary to pre-treat the article with a halogen atom-containing organic solvent such as 1,1,1-trichloroethane and then apply an under coat of a primer thereto, or to subject it to plasma-irradiation, prior to the application of a top coat in order to ensure sufficient adhesive strength of the coated film.
However, the conventional methods as described above require a large number of steps and it takes a long time for practicing these steps. Moreover, it has been feared that the halogen atom-containing organic solvents used in the pre-treatment may adversely affect the human body and environments and accordingly a countermeasure therefore must be urgently considered. For this reason, there has been required the improvement of the coating method conventionally employed while also taking into consideration time and expenses required for the treatments of waste liquor generated.
There have been proposed methods for improving the coating properties of molded articles formed from compositions mainly comprising polypropylene resins and elastomers which comprise adding, to the composition, a diene polymer having terminal hydroxyl groups or a hydrogenated product thereof [see, for instance, Japanese Unexamined Patent Publication (hereinafter referred to as "J.P. KOKAI") Nos. Hei 3-157168, Hei 4-77567, Hei 4-93330 and Hei 3-277636]. The purpose of these methods is to apply a coating without application of any primer coating and/or plasma-irradiation. However, when an article to be coated is not degreased and washed with 1,1,1-trichloroethane, various problems arise in more severe evaluation of coating properties of the resulting coated article. For instance, the coating film is insufficient in resistance to solvent and the coated film near gate portions is peeled off in the X-shaped cross hatch mark test.
The so-called X-shaped cross hatch mark test has been practiced as a means for evaluating the adhesion of a coated film. There have been known the normal cross hatch mark test and more strict X-shaped cross hatch mark test. The normal cross hatch mark test comprises making cuts on the coated film so that two sets of parallel lines which are perpendicular (90.degree.) to one another are formed. On the other hand, the X-shaped cross hatch mark test comprises making cuts on the coated film so that two sets of parallel lines cross one another in the X-shaped form, i.e., at an angle of 45.degree.. In case of the conventional coated articles which are pre-treated with 1,1,1-trichloroethane, adequate results are obtained in the X-shaped cross hatch mark test. On the other hand, in the case of those which are not pre-treated with the organic solvent, adequate results are obtained in the normal cross hatch mark test, but insufficient results are often obtained in the more strict X-shaped cross hatch mark test. Accordingly, there has been desired to provide coated articles which are not pre-treated with 1,1,1-trichloroethane and can provide good results even in the X-shaped cross hatch mark test.
Moreover, it has been a recent tendency, in North America, South America, Europe or the like to use a so-called gasohol which is a mixture of gasoline for automobiles and lower alcohols such as methanol, ethanol and butanol for the purposes of saving natural resources and reduction of cost. The gasohol exerts severe influence on the peeling properties of coating films as compared with the gasoline per se and, therefore, there has been desired for the improvement of these polypropylene molded article in the resistance to gasohol.